Fix chart label clipping, persist speed history across tabs, fix pause button

- Dynamic left padding based on measured label width (no more cut-off numbers)
- Speed history ref lifted to App so chart data survives tab switches
- Pause button uses optimistic UI update for instant visual feedback

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "real-debrid-downloader",
-  "version": "1.5.0",
+  "version": "1.5.1",
   "description": "Real-Debrid Downloader Desktop (Electron + React + TypeScript)",
   "main": "build/main/main/main.js",
   "author": "Sucukdeluxe",